I've never heard the term "dirtbag runner." I remember the term "running bum," which was generally an unemployed or extremely underemployed guy in his twenties (sometimes older) who basically just ran and often ate, slept, and hung out with similar guys, sometimes in packed vans or cars. In the '70s and early '80s, there were tons of them. Maybe they had part-time jobs selling running shoes, waiting on tables, running races with prize money or under-the-table appearance fees, acting as coaching or teaching assistants, helping to put on races, or acquiring crummy little shoe "sponsorships." But they didn't want (or, apparently, need) much more. Some of them, like Frank Shorter and Bill Rodgers, made millions (yes, literally millions) as amateur athletes. Most, I assume, the financial rewards of running to be approximately nonexistent and eventually found more gainful employment.
The number may have peaked with the 1983 Boston marathon. I sensed that it cratered beginning about a year later, when shoe companies cut contracts of good local runners (and runners who didn't make the '84 Olympic team) and put more money into a few athletes who could realistically make an Olympic team and compete with more talented international runners. Also, with the introduction of over-the-table payments, more really good runners, from places like East Africa, started to show at races to pick up the crumbs. Races did start offering more financial incentives for American runners, but to me, it simply emphasized how far the competitive side of this sport had fallen.
Ski bums, I think, do much better.